Newly synthesized eukaryotic mRNA molecules, known as primary transcripts or pre-mRNA are processed before translation. Processing of the pre-mRNAs includes addition of a 5′ methylated cap and an approximately 200-250 base poly(A) tail to the 3′ end of the transcript. Processing of mRNA from pre-mRNA also frequently involves splicing of the pre-mRNA, which occurs in the maturation of 90-95% of mammalian mRNAs. Introns (or intervening sequences) are regions of a pre-mRNA (or the DNA encoding it) that are not included in the coding sequence of the mature mRNA. Exons are regions of a primary transcript that remain in the mature mRNA. The exons are spliced together to form the mature mRNA sequence. Splice junctions are also referred to as splice sites with the 5′ side of the junction often called the “5′ splice site,” or “splice donor site” and the 3′ side the “3′ splice site” or “splice acceptor site.” In splicing, the 3′ end of an upstream exon is joined to the 5′ end of the downstream exon. Thus the unspliced pre-mRNA has an exon/intron junction at the 5′ end of an intron and an intron/exon junction at the 3′ end of an intron. After the intron is removed, the exons are contiguous at what is sometimes referred to as the exon/exon junction or boundary in the mature mRNA. Cryptic splice sites are those which are less often used but may be used when the usual splice site is blocked or unavailable. Alternative splicing, defined as the splicing together of different combinations of exons, often results in multiple mRNA transcripts from a single gene.
Up to 50% of human genetic diseases resulting from a point mutation result in aberrant pre-mRNA processing. Such point mutations can either disrupt a current splice site or create a new splice site, resulting in mRNA transcripts comprised of a different combination of exons or with deletions in exons. Point mutations also can result in activation of a cryptic splice site or disrupt regulatory cis elements (i.e. splicing enhancers or silencers) (Cartegni et al., Nat. Rev. Genet., 2002, 3, 285-298; Drawczak et al., Hum. Genet., 1992, 90, 41-54). Antisense oligonucleotides have been used to target mutations that lead to aberrant splicing in several genetic diseases in order to redirect splicing to give a desired splice product (Kole, Acta Biochimica Polonica, 1997, 44, 231-238).
Antisense compounds have also been used to alter the ratio of naturally occurring alternate splice variants such as the long and short forms of Bcl-x pre-mRNA (U.S. Pat. No. 6,172,216; U.S. Pat. No. 6,214,986; Taylor et al., Nat. Biotechnol. 1999, 17, 1097-1100) or to force skipping of specific exons containing premature termination codons (Wilton et al., Neuromuscul. Disord., 1999, 9, 330-338). U.S. Pat. No. 5,627,274 and WO 94/26887 disclose compositions and methods for combating aberrant splicing in a pre-mRNA molecule containing a mutation using antisense oligonucleotides which do not activate RNAse H.
Proximal spinal muscular atrophy (SMA) is a genetic, neurodegenerative disorder characterized by the loss of spinal motor neurons. SMA is an autosomal recessive disease of early onset and is currently the leading cause of death among infants. The severity of SMA varies among patients and has thus been classified into three types. Type I SMA is the most severe form with onset at birth or within 6 months and typically results in death within 2 years. Children with type I SMA are unable to sit or walk. Type II SMA is the intermediate form and patients are able to sit, but cannot stand or walk. Patients with type III SMA, a chronic form of the disease, typically develop SMA after 18 months of age (Lefebvre et al., Hum. Mol. Genet., 1998, 7, 1531-1536).
The molecular basis of SMA is caused by the loss of both copies of survival motor neuron gene 1 (SMN1), which may also be known as SMN Telomeric, a protein that is part of a multi-protein complex thought to be involved in snRNP biogenesis and recycling. A nearly identical gene, SMN2, which may also be known as SMN Centromeric, exists in a duplicated region on chromosome 5q13 and modulates disease severity. Expression of the normal SMN1 gene results solely in expression of survival motor neuron (SMN) protein. Although SMN1 and SMN2 have the potential to code for the same protein, SMN2 contains a translationally silent mutation at position +6 of exon 7, which results in inefficient inclusion of exon 7 in SMN2 transcripts. Thus, the predominant form of SMN2 is a truncated version, lacking exon 7, which is unstable and inactive (Cartegni and Krainer, Nat. Genet., 2002, 30, 377-384). Expression of the SMN2 gene results in approximately 10-20% of the SMN protein and 80-90% of the unstable/non-functional SMNdelta7 protein. SMN protein plays a well-established role in assembly of the spliceosome and may also mediate mRNA trafficking in the axon and nerve terminus of neurons.
Antisense technology is an effective means for modulating the expression of one or more specific gene products, including alternative splice products, and is uniquely useful in a number of therapeutic, diagnostic, and research applications. The principle behind antisense technology is that an antisense compound, which hybridizes to a target nucleic acid, modulates gene expression activities such as transcription, splicing or translation through one of a number of antisense mechanisms. The sequence specificity of antisense compounds makes them extremely attractive as tools for target validation and gene functionalization, as well as therapeutics to selectively modulate the expression of genes involved in disease.
Certain antisense compounds complementary to SMN2 are known in the art. See for example, WO 2007/002390; U.S. 61/168,885; Hua et al., American J. of Human Genetics (April 2008) 82, 1-15; Singh et al., RNA Bio. 6:3, 1-10 (2009). Certain antisense compounds and methods disclosed herein posses desirable characteristics compared to such compounds and methods known in the art. Chimeric peptide nucleic acid molecules designed to modulate splicing of SMN2 have been described (WO 02/38738; Cartegni and Krainer, Nat. Struct. Biol., 2003, 10, 120-125).